STORM WATER MANAGEMENT REPORT

Silvercreek Junction STORM WATER MANAGEMENT REPORT Howitt Creek at the Silvercreek Parkway Site Guelph, Ontario August, 2008

TSH File 22304A-04 August 19, 2008 STORMWATER MANAGEMENT REPORT Howitt Creek at the Silvercreek Parkway Site Guelph, Ontario 1.0 Background A review of the flows through Howitt Creek has been requested by the City of Guelph Engineering Department as part of the approval process for the Silvercreek Development (formerly the Lafarge Site). TSH was retained to determine design flows for Howitt Creek from upstream lands and for the preliminary design of a stormwater management facility to control these flows to the capacity of the downstream systems. This report summarizes the analysis of the upstream drainage area and presents the preliminary design to the proposed stormwater management facility. Howitt Creek runs through the east side of the development site, and it is proposed to add an attenuation pond to the east of the creek to contain the large storm-event flows. Howitt Creek is fed by the stormwater runoff from its tributary, conveyed through ditches and storms sewers to where the system outlets on the northern limit of the Silvercreek Development site. The system, called the Alma Street Relief Sewer, collects water from a catchment area of approximately 242.3 ha. The catchment area is generally encompassed by Woodlawn Road, Edinburgh Road, the Guelph CNR [GEXR rail line] and Silvercreek Parkway North. The flow from the outlet into Howitt Creek has been the subject of a number of studies. The two most notable studies were completed by Cumming Cockburn Limited and by PVA Consultants (December 2006). Each of these studies estimated substantially different flows through the creek. The Stormwater Management Analysis - Silvercreek Parkway Site by CCL found that during the 100 year recurrence interval, 26.5 m 3 /s would outlet from the Alma Street Relief Sewer into Howitt Creek. CCL modelled the catchment area using OTTHYMO and calibrated their model with information from a monitoring program. In their Addendum to EIS Additional Information Environmental and servicing Reports of May 2006, PVA assumes the flow to Howitt Creek is restricted to the full flow capacity of the Alma Street Relief Sewer, which was calculated to be 14.2 m 3 /s. The primary underlying assumption being that the runoff from the upstream lands cannot enter the storm drainage system at a rate sufficient to surcharge the storm sewers. Geographic information system mapping of the drainage area, consisting of aerial photography, road network, 0.5m interval contours, ditches, storm sewer and catchbasin layout, along with road plan and profile drawings were provided by the City for this review. In addition to the two reports identified above, background information was also obtained from the 1970 Kilborn Engineering report titled Feasibility Report on Northwest Drainage Basin Storm Relief Sewer. Approximately 150m south of the railroad, Howitt Creek flows through twin 1200mm diameter culverts that provide an access road to the east part of the site. The CCL report identified that this structure limits downstream flows to 16 m 3 /s with the balance (10.5 m 3 /s) being conveyed overland, through existing culverts across the Hanlon Expressway to the Northwest Storm Drain outlet. With the current development plan, this flow diversion will no longer be feasible and stormwater attenuation will be required to limit the flows downstream of the development. Page 1

2.0 Tributary Area 2.1 Terrain The land in the tributary area slopes from Woodlawn to the railway. The railway tracks are built up and act as a barrier to the overland drainage path, preventing the water from flowing directly south of the railway tracks. The catchment area has two areas of high elevations along Western and Rosewood Avenues to Paisley Road and along Edinburgh, creating a low lying area between them, along Alma and Guelph Streets. This low area extends from north of Willow Road to Paisley Avenue and into Goldie Park, immediately north of the railway tracks. This low lying area contains sags in the roads and according to the contour lines, areas from which there is no overland flow route. 2.2 Stormwater Management within Tributary Area The area within the tributary area is a mixture of industrial/commercial and residential properties with the majority of the industrial commercial areas located at the north part of the drainage area. Research into the City of Guelph s Planning and Engineering files found 10 properties with SWM practices of the 42 properties reviewed. There was one property that indicated its SWM controls would be implemented in the future, however there is no indication that this has happened. St Joseph s Healthcare, on Westmount Road, has an extensive SWM facility, but it outlets to Westmount Road, which is not apart of the Howitt Creek tributary area. There is, however, an emergency spillway for the facility that outlets during large storm events to Edinburgh Road and into the tributary area. 2.3 Storm Sewer System The main components to the Alma Street Relief Sewer are a 2400mm (96 ) diameter storm sewer along Guelph Street and an 1800mm (72 ) diameter on Alma Street. These sewers combine to enter a 3500mm (138 ) diameter pipe, which is reduced to 2550mm (102 ) diameter pipe as it passes under the railway tracks and outlets to Howitt Creek. Upstream of Alma Street, the sewer extends to Dawson Road, where it collects the stormwater from the roadside ditches. The ditches along Dawson Road are typically greater than 1.5m deep. The Guelph Street sewer is extended through park area to south of Speedvale Avenue West. The drainage area includes more than 400 catchbasins. 3.0 Discussion of Previous Reports The aerial photography shows high impervious percentage at the north part of the watershed. Due to the age of the development, it is believed that a majority of these properties were developed prior to SWM controls being a City requirement (10 of 42 properties have some type of control). The residential area includes generally smaller lots. Given the high impervious percentage, and smaller lot size, the Rational Method runoff coefficient inferred by PVA s analysis of 0.20, appears to be too low unless there is significant surface ponding within the upstream drainage area. Typical residential runoff coefficients are in the 0.35 to 0.45 range and industrial areas are typically 0.6 to 0.9. This means that significantly more water is likely running off the land but is assumed to be not entering the storm sewer system. There are over 400 catchbasins connected to the storm sewer system. According to MTO Drainage Manual charts, each catchbasin on a sloped roadway admits up to approximately 0.06cms into the storm sewer. A catchbasin in a sag, where water can pond above it, can admit approximately 0.30cms into the storm sewer. If 400 catchbasins each admit 0.06 cms into the storm sewer the resulting flow would be 24cms. Given the information above, there would need to be significant surface storage for PVA s assumed flow to be valid. With this surface storage, it is expected that many of the catchbasins would have water ponded over them and could admit much more than 0.06cms into the storm sewers. Page 2

At the upper end of the drainage area, the ditches have sufficient depth to create a hydraulic head on the upper reaches of the storm sewer system. This hydraulic head, coupled with the runoff admitted to the system from the catchbasins would most likely be sufficient to produce a surcharge on the downstream storm sewers. The surcharge would result in flows greater than the full flow condition assumed in PVA s analysis. The OTTHYMO model in the CCL analysis used large subcatchments and did not include any piped flow components. The model was calibrated using relatively small rainfall events captured in the 1991 monitoring program. Through discussions with City Staff and the Developer, it was decided that an independent analysis of the upstream drainage area, using the MIDUSS model, would be prepared. 4.0 Stormwater Management Analysis 4.1 Watershed Analysis Using the available background information and a field reconnaissance tour, the upstream drainage area was defined to be 242.3ha. The drainage area upstream of the development was subdivided into 16 catchment areas using the topographic and storm sewer data. The developed part of the site will be designed to convey runoff to the Northwest Storm Drain and away from Howitt Creek, so this area was not included in the model. The existing creek valley and the lands east of the creek, where the proposed stormwater management facility is located, were added to the analysis so that the flow in Howitt Creek leaving the site could also be determined. Areas and approximate volumes of upstream ponding were identified using the topographic information and plan and profile drawings. These areas include the ditches on Dawson Road and ditches, pipe and surface ponding in Norm Jary Park and along Alma Street. MIDUSS modeling parameters, including area, impervious percentage, gradient and length were established for each of the subcatchment areas. The impervious percentage for subcatchments that included properties with known on-site stormwater management facilities was reduced to account for the on-site attenuation. Appendix A includes the design parameters used in the model for each subcatchment area. Figure 1 shows the catchment areas, properties that include on-site stormwater management facilities and areas of potential ponding used in the model. 4.2 Runoff Modeling City of Guelph standard rainfall parameters were used in the analysis and are summarized in Appendix A. Figure 1 includes the MIDUSS model schematic used in the analysis. A comprehensive model that includes all upstream storm sewers is beyond the required scope of this review, however, the model includes equivalent pipe sections to account for the conveyance through the storm sewer network, diversions where the piped flow capacity is exceeded and temporary storage of runoff in the areas described above. Flows from each subcatchment area were calculated and where appropriate, routed through the temporary ponding/storage areas and the piped network. Where the model predicted that the existing piped system would surcharge, the topographic and plan and profile drawings were reviewed to determine whether the amount of expected surcharge would be accommodated within the system. The resulting flows entering Howitt Creek from the Alma Relief Sewer are 11.28 m 3 /s and 27.18 m 3 /s for the 5 year and 100 year storms respectively. MIDUSS output files are included in Appendix C and model results on a reach by reach basis are summarized in the schematic drawing included in Appendix D. Page 3

The model then routes this flow through the channel downstream of the railroad and adds in the flow from the eastern part of the development. The resulting flow to the proposed stormwater management facilty is 11.65 m 3 /s and 28.03 m 3 /s for the 5year and 100 year storms respectively. 4.3 Downstream Constraints Both the 1991 CCL study and the 1970 Kilborn study identified the culvert that carries Howitt Creek under Waterloo Avenue as the critical conveyance constraint. According to the 1970 report and as confirmed with drawings provided by the City, this structure is a concrete box with a width of 2.44m and a height of 2.13m. It is approximately 91m long and is constructed at a 2.2% grade. Using MTO Design Charts, we have calculated that the capacity of this structure is 13 m 3 /s with the upstream water level at the obvert of the culvert and 14.9 m 3 /s with a water level at the elevation of the top of the structure. The velocity of flow through the structure at this flow is very high (greater than 7 m/s). This confirms the capacity information provided in the previous studies. As indicated in the CCL report, the existing twin 1200mm diameter culverts on site do no have sufficient capacity to convey the flows from the Alma Relief Sewer. The capacity of these culverts will result in flooding upstream of the structures. As the water level rises upstream, some runoff is diverted to the Northwest Storm Drain and once the internal access road is overtopped, the remainder is conveyed downstream through Howitt Creek. The CCL report reported that the peak flow being conveyed downstream of the development is 16 m 3 /s and that the lands downstream of the development but upstream of Waterloo Avenue contribute an additional 3 m 3 /s. The resulting flow to the Waterloo Avenue culvert results in periodic flooding upstream of the structure. CCL recommended that the flow conveyed through the development be limited to 11m 3 /s to reduce the potential flooding impact. 4.4 SWM Facility Design In order to limit the Howitt Creek flows to suit the capacity of the Waterloo Avenue culvert, a stormwater management facility is proposed on the eastern part of the development site. Figure 2 shows the proposed stormwater management facility design. The existing twin 1200mm diameter culverts will be removed and a new twin 1200mm diameter crossing provided approximately 90m downstream of the current location. The lands east of the creek will be excavated to provide temporary storage of the upstream flows. The flow rate through the culvert is dependent on the water level upstream of the culvert. As the water level rises upstream of the new culverts, additional storage becomes available and there is an increased flow through the structure. A stage-storage-discharge chart for the facility was calculated and is provided in Appendix B. The resulting flood levels for the 25mm storm, 5 year and 100 year events are shown in Figure 2. Results of the modeling are summarized in Table 1. Table 1 Proposed Stormwater Management Facility Performance Design Storm 25mm 5 Year 100 Year Peak Inflow (m 3 /sec) 3.95 11.65 28.03 Peak Outflow (m 3 /sec) 3.79 7.63 10.82 Maximum Depth (m) 1.179 2.259 3.764 Maximum Storage (m 3 ) 643 6,010 35,766 Maximum Elevation (m) 318.68 319.76 321.26 Page 4

5.0 Conclusions The Alma Relief Sewer discharges at a maximum flow of 27.18 m 3 /s to Howitt Creek at the upstream limit of the Silvercreek Junction development. There are no flows to Howitt Creek from the portion of the Silvercreek Junction site that is proposed for development. The existing detention storage located in ditches, pipes and surface storage within the upstream drainage area attenuates existing flows reaching the Silvercreek Junction site. Any future development or drainage improvements within the upstream lands must take into consideration the flow capacity of the downstream systems and implement additional stormwater management features as required. A stormwater management facility provided on the Silvercreek Junction site can control flows from the upstream lands to a level suitable for the downstream conveyance constraint at Waterloo Avenue. Peak flows being conveyed downstream of the development will be reduced from 16 m 3 /s to 10.8 m 3 /s. Provision of a stormwater management facility on the Silvercreek Junction site will remove a flow of approximately 10.5 m 3 /s from the Northwest Storm Drain. All of which is respectfully submitted; totten sims hubicki associates Rick Clement, P.Eng. Manager, Municipal Engineering Page 5